SK-54 is a curious fossil. The 1.5 million year old skullcap represents a juvenile Paranthropus robustus, one of the heavy-jawed hominins which lived in prehistoric South Africa, but there is something that makes this skull fragment particularly special. Near one of the sutures along the back of the skull are two neat puncture marks, the hallmark of a leopard.
Even though it was initially proposed that SK-54 had been murdered by another australopithecine wielding a weapon of bone or horn, in the late 1960′s the paleontologist C.K. Brain was able to demonstrate that the holes almost precisely fitted with the lower canine teeth of a leopard. What’s more, Brain determined that many of the accumulations of bones found in the South African cave deposits were attributable to the activities of predators, meaning that for a long span of time our ancient relatives (particularly juveniles) may have regularly been cat food. Nor were hominins the only primate fossils to be found in these accumulations. It seems that the prehistoric leopards had a taste for primates, just as some living leopards do.
In the Tai Forest of Cote d’Ivoire, leopards frequently kill and consume primate prey. It is rare that scientists observe an attack in progress, but the primate remains in big cat scat confirm that primates are a major part of the leopard diet in this forest. Along with what we know from the fossil record, these prey preferences have raised an interesting question. Have the hunting habits of leopards influenced primate evolution?
It has previously been proposed that predation has caused some primates to evolve larger body size, larger social groups, groups with more males in them, and other characteristics, and to test these ideas scientists Klaus Zuberbuhler and David Jenny used radio collars to monitor the movements of the Tai leopards and checked leopard droppings for signs of any of nine primate species living in the forest (specifically red colobus, black-and-white colobus, olive colobus, Diana monkey, lesser spot-nosed monkey, Campbell’s monkey, putty-nosed monkey, sooty mangabey, and chimpanzees). By putting this information together with what was known of the natural history of each primate species, the scientists could then see if some hypotheses about the influence of predation on primate evolution held up. For instance, if the hypothesis that predation had driven large body size it would be expected that leopards would prefer smaller species, and if larger social groups were a response to predation it would be assumed that leopards would prefer primates which gathered in smaller groups or were solitary more often.
After looking at the movements and fecal samples of four Tai leopards over the course of two years (1992-1994), what the researchers found was that the big cats primarily hunted during the day, and during that time they often searched out monkey groups to stalk. That they did succeed in catching monkeys was confirmed through the 200 scat samples. The majority of prey traces in the feces were from small mammals, particularly small hoofed animals called duikers, but monkey remains were also common (about 64 occurrences, while chimpanzee remains occurring only once). Overall, this was comparable to the results from a similar study published in 1984, with the only difference being that comparatively more Diana monkeys and fewer colobus monkeys were found in the earlier study, and this was probably attributable to known fluctuations in population during that time.
With this data in hand, Zuberbuhler and Jenny could test the leopard’s prey preferences against some of the previously-published hypotheses about how predation has affected primate evolution. They were surprised by the results. Leopards appeared to prefer larger monkeys over smaller ones, so it did not seem that larger body size afforded any protection from predators. Likewise, monkeys which lived in larger groups or in groups which had a larger proportion of adult males were preyed upon more often, again running counter to expectations.
The reasons for the disparity between the expectations and the habits of the Tai leopards are difficult to discern. While large body size might make a monkey more susceptible to a leopard due to lessened agility, large body size might be an effective deterrent against the birds of prey which also hunt monkeys in the forest. As for group size, whatever vigilance benefits come from being in a large group might be counterbalanced by the fact that larger groups of primates are easier to find and thus might be preferred by leopards. Rather than providing protection, the traits which were supposed to insulate the monkeys from predation might make them more vulnerable.
But what about the chimpanzees? Why, if leopards prefer larger prey in easy-to-locate groups, were chimpanzees almost absent from the sample? Perhaps they are just to large and strong to be eaten. Leopards, though powerful, are relatively small as big cats go, and they would certainly risk severe injuries if they chose to attack a healthy adult chimpanzee. In fact, at least one of the leopards tracked avoided chimpanzee groups, suggesting that the apes are a special case.
In documentaries, textbooks, and other popular accounts predation is often shown as having a major influence on the evolution. Nothing so poignantly sums up the “struggle for existence” as images of a pride of lions taking down a zebra, a Nile crocodile snapping at wildebeest in the Mara River, a spider wrapping up a fly, or a great white shark closing its jaws on a seal. Yet, as the study of leopards in the Tai Forests suggests, the actual influence of predation on prey species may not be as simple as an arms race of increasing speed, larger body size, or other characteristics outlined in popular examples. Traits thought to protect prey species may, in some cases, prove to be liabilities, and so greater attention must be paid to who predators are hunting and why.
This is not to say the leopards and other predators have had no effect on primate evolution whatsoever, of course. Primates, including our own ancestors, have had to face numerous predators during the course of evolutionary history, and the fact that many primates have evolved anti-predation strategies (such as predator-specific alarm calls) illustrates that predation precipitates some change. No doubt predation had some influence on our own ancestors, too, and I can only imagine how prehistoric hominins, wandering a landscape populated by a wider diversity of carnivores than is seen today, coped with the presence of so many dangerous animals.
Zuberbühler, K.; Jenny, D. (2002). Leopard predation and primate evolution Journal of Human Evolution, 43 (6), 873-886 DOI: 10.1006/jhev.2002.0605